A sensor device is known in which its electrical characteristics change with a change of temperature. In the construction of electrically heatable blankets it has been known to provide an electrical resistance heating element which had an insulation which incorporated a layer of sensor material having a negative temperature coefficient of electrical impedance and this was used as part of a safety cut-out control system to prevent overheating of the blanket. I have taken this device and have used it as a fire detecting means. For example the device, which may be in the form of a wire or cable, can be positioned at suitable locations to detect fire. It may for example be run along and lie adjacent to electrical power transmitting cables in a power station or the like to sense any overheating of the power transmitting cables. In order to be able to use such a device as a fire detecting device I have now devised the detecting apparatus which detects any change in the electrical characteristics of the sensor device.